Owing to the increasing scarcity of non-renewable energy reserves such as coal, petroleum and uranium, increased use is being made of alternative nondepletable energy sources, such as photovolatic energy. Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices, have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices has been limited by problems associated with the manufacture thereof. More particularly, single crystal semiconductor alloy materials (1) are difficult to produce in sizes substantially larger than several inches in diameter; (2) are thicker and heavier than their thin film counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable effort has been expended to develop systems and processes for preparing thin film amorphous semiconductor alloy materials which encompass relatively large areas and which can be deposited so as to form p-type and n-type semiconductor alloy layers for the production therefrom of thin film electronic devices, particularly thin film p-n type and n-i-p type photovoltaic devices which are substantially operatively equivalent or superior to their crystalline counterparts. It should be noted at this point that the term "amorphous" as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occurs. It is to be noted that pursuant to the foregoing definitions, the microcrystalline n and p-doped semiconductor alloy materials referred to herein fall within the generic term "amorphous".
Amorphous thin film semiconductor alloys have gained acceptance as the material from which to fabricate electronic devices such as photovoltaic cells, photoresponsive and photoconductive devices, transistors, diodes, integrated circuits, memory arrays and the like. This is because the amorphous thin film semiconductor alloys (1) can be manufactured by relatively low cost continuous processes, (2) possess a wide range of controllable electrical, optical and structural properties and (3) can be deposited to cover relatively large areas. Among the semiconductor alloy materials exhibiting the greatest present commercial significance are amorphous silicon, germanium and silicon-germanium based alloys. Such alloys have been the subject of a continuing development effort on the part of the assignee of the instant invention, said alloys being investigated and utilized as possible candidates from which to fabricate a wide range of semiconductor, electronic and photoresponsive devices.
Additionally, said assignee has developed commercial processing systems for the continuous roll-to-roll manufacture of large area photovoltaic devices. Such continuous processing systems are disclosed in the following U.S. patents, disclosures of which are incorporated herein by reference: U.S. Pat. No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 4,410,588, for Continuous Amorphous Solar Cell Production Systems; and U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition and Isolation System And Method. As disclosed in these patents a web of substrate material may be continuously advanced through a succession of interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor alloy material onto the web or onto a previously deposited layer. In making a photovoltaic device, for instance, of n-i-p type configurations, the first chamber is dedicated for the deposition of an n-type semiconductor alloy material, the second chamber is dedicated for the deposition of a substantially intrinsic amorphous semiconductor alloy material, and the third chamber is dedicated for a deposition of a p-type semiconductor alloy material. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to, photovoltaic devices which include one or more n-i-p type cells. By making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained. Note, that as used herein the term "n-i-p type" will refer to any sequence of n and p or n, i and p semiconductor alloy layers operatively disposed and successively deposited to form a photoactive region wherein charge carriers are produced by the absorption of photons from incident radiation.
The concept of utilizing multiple stacked cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed were limited to the utilization of p-n junctions formed by single crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In tne first cell, a large band gap material absorbs only the short wavelength light, while in subsequent cells, smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be economically fabricated in large areas by employing thin film amorphous, semiconductor alloy materials (with or without crystalline inclusions), in accordance with the principles of the instant invention. It should be noted that Jackson employed crystalline semiconductor materials for the fabrication of his stacked cell structure; however, since it is virtually impossible to match lattice constants of differing crystalline materials, it is not possible to fabricate such crystalline tandem cell structures in a commercially feasible manner. In contrast thereto, and as the assignee of the instant invention has shown, such tandem cell structures are not only possible, but can be economically fabricated over large areas by employing the amorphous semiconductor alloy materials and deposition techniques described herein.
In U.S. Pat. No. 4,440,107, filed July 12, 1982, entitled Magnetic Apparatus For Reducing Substrate Warpage, assigned to the Assignee of the instant invention and the disclosure of which is incorporated herein by reference, it was noted that the deposition of successive layers of semiconductor alloy material upon a web of substrate material which was adapted to continuously move through a succession of isolated, but operatively interconnected deposition chambers developed specific problems. In particular, it was noted that in order to deposit semiconductor alloy material from which to fabricate highly efficient photovoltaic devices, it is necessary that each one of the successive layers of semiconductor alloy material (deposited in each respective one of the successive chambers) be of substantially uniform thickness and material properties. However, prior to the disclosure of said U.S. Pat. No. 4,440,107 patent, such continuous deposition apparatus was unable to prevent (1) transverse warping (warping is used synomously to connote buckling or canoeing) of the web of substrate material caused by the force of gravity acting upon the web, the elongated path of travel which the web of substrate material follows, stresses from external sources developed upon the web of substrate material, the high deposition temperatures to which the web of substrate material is continuously subjected, and the forces created by the highly stresssed semiconductor alloy material deposited upon the web of substrate material; or (2) longitudinal warping of the web of substrate material which occurs when the tension on the web of substrate material initiated by the drive and tensioning motors is not precisely adjusted. Obviously, when the web of substrate material warps (either transverse to or longitudinal to the direction of web movement), peaks and valleys are formed upon the deposition surface thereof. It then becomes commonplace to have greater thicknesses of semiconductor alloy material deposited in the valleys while none or lesser thicknesses of the semiconductor alloy material are deposited at the peaks. Such non-uniform deposition of the semiconductor alloy material is undesirable, can create short circuit paths, can create nonuniform electrical, optical and compositional variations in properties of the material, and generally serves to impair the photoconversion efficiency and operation of large area photovoltaic devices fabricated therefrom.
Accordingly, the inventive concept disclosed in said U.S. Pat. No. 4,440,107 patent, i.e., to substantially reduce the transverse and longitudinal warpage of the web of substrate material traveling through the operatively interconnected deposition chambers, was to establish in each of said chambers at least one magnetic force which flattened the web of substrate material by urging said web into a substantially planar configuration as it passed through the deposition region developed therewithin. The flattening of the web of substrate material created by the magnetic field permitted the substantially uniform deposition of successive layers of semiconductor alloy material onto the web of substrate material and, hence, was responsible for an increase in the overall efficiency of large area photovoltaic devices fabricated therefrom.
More specifically, and in order to carry out this stated objective, the web of substrate material was urged upwardly out of its normal path of travel by a plurality of substantially equally spaced rows of ceramic magnets, each magnet extending substantially across the entire transverse width of the web of substrate material. In a preferred embodiment disclosed in that patent, the elongated magnets were spacedly arranged throughout the entire length of the vapor deposition processor, at approximately 8 to 10 inch intervals, so that the web was substantially prevented from warping as layers of semiconductor alloy material were deposited thereupon.
While the concept disclosed in said U.S. Pat. No. 4,440,107 was generally successful in preventing warpage of the web of substrate material passing through such a continuous vapor deposition processor, the addition of another mechanism to the processor, which mechanism was adapted to aid in the take-up of the web of substrate material, i.e., the automatic web steering assembly, was responsible for initiating other types of warpage-related problems, which problems were aggravated if not caused by the magnetic field described hereinabove. More particularly, U.S. Pat. No. 4,485,125, filed Jan. 24, 1983, entitled Method For Continuously Producing Tandem Amorphous Photovoltaic Cells, assigned to the Assignee of the instant invention and the disclosure of which is incorporated herein by reference, describes one type of automatic web steering assembly. The purpose of the automatic web steering assembly was to sense and correct the position of the advancing web of substrate material so as to assure the proper tracking of the web through the multiple deposition chambers of the processor and finally, to assure uniform wind-up of that web onto the take-up core. Generally, the web of substrate material was steered in said U.S. Pat. No. 4,485,125 patent by changing the tension at one edge thereof relative to the opposite edge by means of a specifically designed steering roller.
In operation, the web steering mechanism is located proximate the take-up roller and includes a photosensor assembly which senses the position of one edge of the advancing web of substrate material and generates an electrical signal indicative of that position. The signal is communicated by the photosensor to a motor controller which rotates a servo-motor in either a clockwise or counter-clockwise direction depending upon the position of said one edge of the web relative to a home position maintained on the take-up roller. The rotational motion is communicated to a drive train of the steering mechanism, said mechanism then initiates rotational motion which is adapted to pivot the axis of rotation of the steering roller. The rotational motion changes the tension on one of the longitudinal edges of the web, thus steering the web and eliminating "telescoping" as the web is wound about the take-up core.
Referring now to FIGS. 3 and 4, the manner in which the magnetic field established by the uniformly spaced, elongated magnets responds to a steering action by the web steering mechanism in order to place one longitudinal edge of the web of substrate material in tension and the opposite longitudinal edge of the web of substrate material in compression, thereby buckling that edge of the web which is held in compression, will now be described.
FIG. 3 illustrates a web of substrate material 11 assuming a normal path of travel through the longitudinal extent of the vapor deposition processor. The web of substrate material 11 is adapted to be held in a generally planar configuration for the deposition of a semiconductor alloy material thereonto by a plurality of spaced, elongated, transversely extending ceramic magnets 50 so as to prevent buckling of that web during the deposition process. In FIG. 3, the web of substrate material 11 is depicted as assuming a substantially perfect alignment in which (1) the lateral edges of the web of substrate material 11 are perfectly aligned and (2) the planar surface of the web of substrate material is held in a single plane throughout the path of travel of that web through the processor.
In contrast thereto and with particular reference to FIG. 4, the longitudinal edges of the web of substrate material 11 have been found to assume a generally non-aligned attitude and the deposition surface of the web of substrate material has been found to assume a non-planar attitude as the web moves through the length of the vapor deposition processor. The phantom lines present in FIG. 4 depict a perfect alignment of the longitudinal edges of the web while the solid lines indicate the true condition which is seen as the web of substrate material 11 moves through said processor. This condition has been seen when the processor is opened up after a deposition run in which non-uniform semiconductor alloy material was deposited upon the deposition surface of the web of substrate material 11. It has therefore been noted that buckling of the web of substrate material 11 along one lateral edge thereof can be directly observed. As this phenomena is traced throughout the path of travel of the web through the processor, the web appears to have been locally displaced by as much as one-quarter of an inch at one edge thereof so that the opposite lateral edge begins to buckle. This phenomena of buckling was not prevalent until heavy magnetic rollers were added to the processor.
One plausible explanation for the hereinabove described buckling problem is that the steering mechanism places one lateral edge of the web of substrate material in tension in order to shift the position of that web relative to its home position on the take-up roller. Absent the presence of the large magnetic field which tends to maintain the position of the web constant despite the input of the externally acting steering forces, the web of substrate material would quickly realign itself in the position indicated in FIG. 3. However, because the strong magnetic field is present, the web is unable to move laterally (or "walk"). This anti-steering component of the magnetic field tends to angulate the web and amplifies the tensile force sensed by the opposite longitudinal edge thereof. More particularly, when the web of substrate material encounters the first of the elongated magnets 50a, it angulates due to the resistive forces of that magnet and the magnets immediately upstream thereof. The result is a buckling of that edge of the web of substrate material 11 which is subject to the compressive forces. It is to the end of providing for the corrective lateral displacement of a longitudinally moving web held in a planar configuration to which the inventive concept presented by the instant application is directed.
These and other objects and advantages of the present invention will become apparent from the detailed description, the drawings and the claims which follow.